Added a new method to update the cache of all pages linking to a given page without...

Added a new method to update the cache of all pages linking to a given page without using excessive time or memory. This turns out to be a common operation in MediaWiki, I needed to fix callers in a lot of files. Backwards compatibility is maintained (of course). Also did some cleanup in Article.php, another 3 years of that and it'll be looking pretty. New job type added to the job queue, some refactoring done in JobQueue.php to support it.